SCIENCE OF SOIL

Soil A collection of natural bodies developed in the unconsolidated mineral and organic material on the immediate surface of the earth that serves as a natural medium for the growth of

SCIENCE OF

SOIL

1

Soil

A collection of natural bodies developed in the unconsolidated mineral and organic material on the immediate surface of the earth that serves as a natural medium for the growth of land plants and has properties due to the effects of climate and living matter acting upon parent material, as conditioned by topography, over a period of time.

Primary and Secondary

Minerals

 Primary Minerals: Minerals that have persisted with little change in composition since they were extruded in molten lava(eg quartz, mica and feldspars).They are most prominent in sand and silt fractions.

 Secondary Minerals: Minerals such as the silicate clays and iron oxides, have been formed by the breakdown and weathering of less resistant minerals as soil formation progressed.

Weathering of rocks

It is physical and chemical disintegration and decomposition of rocks Weathering creates the parent material over which the soil formation takes place Later weathering, soil

simultaneously.

5

Soil formation

The mineral weathering combines with the associated physical and chemical phenomena constitute the process of soil formation.

It

includes-1 The addition of organic & mineral materials

2 The loss of these materials from the soil

3 Translocation of materials from one point to Another within the soil column

4 Transformation of minerals & organic substances within the soil

Two Approaches:

9

The origin of the soil ,its classification, and its description are examined in pedology (pedon-soil or earth in greek) Pedology is the study of the soil as a natural body and does not focus primarily on the soli’s immediate practical use A pedologist studies, examines, and classifies soils as they occur in their natural environment.

Composition of soil

Soil Profile and its

Layers(Horizons)

 Examination of a vertical section of a soil as seen

in a roadside cut or in the walls of a pit dug in the field, reveals the presence of more or less distinct horizontal layers Such a section is called a profile, and the individual layers are known as horizons

13

Topsoil and Subsoil

 When a soil is ploughed and cultivated, the natural state of the upper 12-18 centimeters(5-7 inches) is modified This manipulated part of the soil is referred to as the surface soil or the topsoil.

 The subsoil is comprised of those soils layers underneath the top soil.

Mineral (inorganic) and

organic soils

composition, low in organic matter ranges from 1 -6%.

 Organic soils: 50% organic matter by volume (at least 20% by weight).

15

Soil Texture and Soil

Structure

 Soil Texture: Proportions of different sized particles present in soil.

silt and clay particles within the soil.

Table:General properties of three major inorganic soil

Secondary Secondary3.Attraction of particles for each

4 Attraction of particles for water Low Medium High

5.Ability to hold chemical nutrients

and supply them to plants Very low Low High

6.Consistency properties when wet Loose , gritty Smooth Sticky,

plastic 7.Consistency properties when dry Very loose,

gritty Powdery, some clods Hard clods

Soil Organic Matter

 Soil organic matter comprises an accumulation of partially disintegrated and decomposed plant and animal residues and other organic compounds synthesized by the soil microbes as the decay occurs Such material is continually being broken down and re-synthesized by soil microorganisms Consequently, organic matter is a rather transitory soil constituent, lasting for a few hours to several hundred years

 Organic matter binds mineral particles into granules that are largely responsible for the loose easily managed condition of productive soils and increases the number of water a soil can hold

 It is also major soil source of phosphorus and sulfur and the primary source of nitrogen (3 elements essential for plant growth)

 Organic matter, including plant and animal residues, is the main source of energy for soil organisms Without it biochemical activity would come to a near standstill.

 In addition to the original plant and animal residues and to their partial breakdown products, soil organic matter includes complex compounds that are relatively resistant to decay These complex materials, along with some that are synthesized by the soil microorganisms, are collectively known as humus This material is usually black and brown in colour, is very fine(colloidal) in nature.

19

Soil Water

 Water is hold in the soil for varying degree of tenacity depending on the amount of water present and the size of the pores

 Together with its soluble constituents, including nutrient elements(eg Ca, P, N and K), soil water makes up the soil solution, which is the critical medium for supplying nutrients to growing plants

 The movement can be in any direction; downward in response to gravity, upward as water moves to the soil surface to replace that lost by evaporation, and in any direction toward plant roots as they absorb this important liquid Although some of the soil moisture is removed by the growing plants, some remains in the tiny pores and in thin films around soil particles The soil solids strongly attract the soil water and consequently compete for it with plant roots

Soil Solution

 The soil solution contains small but significant quantities of soluble inorganic and organic compounds, some of which contain elements that are essential for plant growth

 Critical property of the soil solution is its acidity or alkalinity Many chemical and biological reactions are dependent on the levels of hydrogen ions and hydroxide ions in the soil These levels influence the solubility, and in turn the availability to plants, of several essential nutrient elements such as Fe, Mn, P,

Zn and Mo.

21

 The concentration of hydrogen(H+) and hydroxide

ions(OH-) in the soil solution is commonly ascertained by determining its pH Technically the pH is the negative

logarithm of the concentration of hydrogen ion in the soil solution Thus each unit change in pH represents a

tenfold change in the activity of the H+ and OH- ions

3-4 4-5 5-6 6-7 7 7-8 8-9 9-10 10-11

Clay and Humus

 The attraction of ions such as Ca2+, Mg2+, and K+ on the surfaces of colloidal clay and humus is not as exciting

as is the exchange of these ions for other ions in the soil solution For example, an H+ ion released to the soil solution by a plant root exchange readily with a potassium ion(K+) adsorbed on the colloidal surface The K+ ion is then available in the soil solution for uptake by the roots of crop plants A simple example of such cation exchange illustrates this point

K+ + H+(aq) H+ + K+(aq)

(adsorbed) (in soil solution) (adsorbed) (in soil solution)

23

colloid colloid

Clay Micelle

-ve charge +ve charge

Ionic double layer

Al Ca

Mg

K

H Na

Essential nutrient element and their sources

Use in relatively

large amounts Use in relatively large amounts Use in relatively small amounts

Mostly from air and

water From soil From soil

Carbon(C ) Nitrogen(N) Iron(Fe)

Hydrogen(H) Phosphorus(P) Manganese(Mn)

Oxygen(O) Calcium(Ca) Boron(B)

Magnesium(Mg) Molybdenum(Mo) Sulfur(S) Copper(Cu)

Zinc(Zn) Chlorine(Cl) Cobalt(Co)

25

Second, soil air generally has a higher moisture content than the atmosphere; the relative humidity

of soil air approaches 100% when the soil moisture is optimum.

Third, carbon dioxide in soil air is often several times higher than the 0.03% commonly found in the atmosphere, Oxygen decreases accordingly and, in extreme cases 5-10%, or even less, as compared to about 20% for normal atmosphere

Composition of soil air

Sandy soil air 79.20 19.95 0.30

Loamy soil air 79.20 19.20 0.62

Clay soil air 79.20 19.69 0.66

Manured soil

air

79.20 18.23 1.85

Soil Degradation

 Soil degradation is a concept in which the

affected by one or more combination of human-induced processes acting upon the land It is viewed as any change or disturbance

to the land perceived to be deleterious or undesirable Natural hazards are excluded as a cause, however human activities can indirectly affect phenomena such as floods and bushfires.

It is estimated that up to 40% of the world's agricultural land is seriously degraded.

The major causes include:

 Land clearance, such as clearcutting and

deforestation

 Agricultural depletion of soil nutrients through poor farming practices

 Overgrazing

 Inappropriate Irrigation and overdrafting

 Urban sprawl and commercial development

 Land pollution including industrial waste



29

 Population pressure

 The major stresses on vulnerable land include:

 Accelerated soil erosion by wind and water

 Soil acidification and the formation of acid sulfate soil

resulting in barren soil

 Soil alkalinisation owing to irrigation with water containing sodium bicarbonate leading to poor soil structure and

reduced crop yields

 Soil salinization in irrigated land requiring soil salinity

control to reclaim the land

 Waterlogging in irrigated land which calls for some form of subsurface land drainage to remediate the negative effects

 Destruction of soil structure including loss of organic matter

 Ultimately results into low vegetation cover, extensive soil erosion which leads towards desertification

31

 Every year 84 billion tonnes of productive top soil are lost world wide through degradation

of land globally (De Man et al 2007).

productive lands are oversalted because of

improper water management.

Soil Erosion

 Soil erosion is the process of detachment of soil

particles from the parent body and transportation

of the detached soil particles by wind or water.

Mechanism of Water Erosion:

Forms of Water Erosion

 Sheet Erosion: uniform removal of top soil in thin layer

from the field, least conspicuous.

 Rill Erosion: channelization begins ,no longer uniform.

 Gully Erosion: unchecked rills result in increased

channelization of runoff.

 Ravines: manifestation of prolonged process of gully

erosion Deepening & Widening of gullies used to form

ravines.

 Landslides: occur in mountain slopes when the slope

exceeds 20 per cent and width 6 m.

 Stream-bank Erosion: Seasonal streams or rivulets often

change their course from season to season due to blockage

of their previous course by transported rocks, clods of soil & vegetation grown during lean periods.

Gully erosion Ravine erosion

37

Forms of wind erosion

soil particles is by suspension Dust particles of fine sand ( less than 0.1 mm dia) are moved parallel to ground surface and upward About 5-15 % of wind erosion afftected soil is transported by this process

lifted by the wind, then fall back to the ground, so they move in a hopping or bouncing fashion These particles

cause abrasion of the soil surface and as they hit other

particles they break into smaller particles, a process

called attrition Depending on conditions, this process

may account for 50-70% of the total movement of soil

(more than 0.5 mm dia) along the surface Surface creep account to 5-25% of total movement due to action of wind

39

Soil Conservation

Definition

Soil conservation is using and managing land based on the capabilities of the land itself.

Soil Conservation Measures

Agronomic Measures

 Contour Cultivation – By ploughing and sowing across the slope, each ridge of plough furrow and each row of the crop act as an obstruction to runoff, providing more opportune time for water to enter into the soil and reduce soil loss

 Tillage – Tillage alters soil physical characters like porosity, bulk density, surface roughness and hardness of pans Conventional tillage includes ploughing twice or thrice followed by some secondary operations like harrowing and planking that smoothen and pack the soil in seed-bed and/or control weeds.

 Mulching – Mulches are any material such as straw, plant residues, leaves, loose soil or plastic film placed

on the soil surface to reduce evaporation, erosion or

to protect plant roots from extremely low or high

Cropping Systems: It represents cropping patterns used on a farm and their interaction with farm resources, other farm enterprises and available technology Cropping pattern indicates yearly sequence and spatial arrangements of crops and fallow in an area Practically,

it implies different crops grown either in combination or sequentially in farm over the years Therefore, growing a crop which produces the maximum cover, in addition to principle crops like rice

or wheat, reduces runoff and soil loss For example, cow pea and green gram are important cover crops for the rainy season Tobacco, being a clean cultivated crop, allows higher runoff and soil loss These losses can be reduced by growing cowpea or green gram during early monsoon before tobacco is planted

(e.g groundnut, beans) are alternated with strips of erosion permitting crops (e.g maize) The strips are laid across the slope

Mechanical Measures

 Contour Bunding – Runoff from any given surface is along the line

of greatest slope and the velocity of runoff increases with the vertical distance through which it is moved The contour bund being on the same elevation, assures that the depth of water against the bund is uniform throughout its length It ensures uniform distribution of water above the bunds and therefore, better cultivation possibilities than any other type of bund As the bunds are at regular intervals, they intercept the runoff from attaining erosive velocity and causing erosion The velocity of flowing water

is slowed down and water thus held on the field for a longer time, soaks into the soils.

 Broad Base Terrace - A terrace is a combination of ridge and channel built across the slope These terraces have wide base and low height of ridge and usually formed with machinery BBTs are constructed in soils with high clay content which develop deep cracks in summer (e.g Black soil).

43

Bench Terracing - Bench terracing consists of transforming relatively steep land into a series of level strips or platforms across the slope of the land It reduces the slope length and consequently erosion The field is made into a series of benches by excavating the soil from upper part of the terrace and filling in the lower part On steeply sloping and undulated land, farming practices is possible only with bench terracing It is usually practiced on slopes ranging from 16 to 33%

Trenching –Contour trenches are made in non-agricultural land for providing adequate moisture conditions in order to raise trees or grass species The trenches are usually 60 cm × 48 cm in size The spacing varies from 10 to 30 m

Vegetative Barriers – these are closely spaced plantations-usually a few rows of grasses or shrubs - grown along contours They act as barrier

to check the velocity of overland flow and entrapment of silt load behind

them Khus (Vettiveria zelanica) is the most suitable plant for this